In a communications system, multiple carriers may operate in a Frequency Division Multiplex (FDM) arrangement, where different transmitters are operated at different (but nearby) frequencies. To reduce the likelihood that the different transmissions interfere with each other, the various components on the transmit side should be operated in a linear manner. If the components are operated nonlinearly, then in general, the output spectrum of a transmitter will “regrow” and interfere with its neighbors (e.g., creating adjacent channel interference (ACI)).
For most of the transmitter design, the requirement for linearity is not difficult to meet. It is usually a matter of operating the various stages of amplification with sufficient backoff so they remain in the linear operation region. For the final amplifier stage (HPA), this is more difficult. To allow enough backoff would mean to buy a more expensive amplifier than is actually needed, and also would result in lower power efficiency, increasing power supply and heat removal costs. The result is that to minimize cost, it is necessary to use an amplifier that is just large enough to operate with some minimal backoff to limit interference. Having done this, the problem becomes that if the drive level to the amplifier (operating point) is not carefully controlled, then the amplifier could still be overdriven resulting in excessive ACI. Therefore, to minimize cost and allow the use of the minimal amplifier, it is critical to control the operating point of the amplifier. Even if the drive level to this amplifier was kept constant, the amplifier itself may change characteristics over temperature and frequency. So, a reliable method of determining the operating point is needed. While it may be feasible in some cases to measure parameters on the transmit side, e.g., amplifier input power, temperature, etc., for a low-cost transmitter this is not feasible.
What is needed, therefore, is an approach to reliably measure the operating point of a transmitter amplifier, without requiring special instrumentation at the transmitter, based on measurements performed at a receiver.